A global project role is a ready-to-use role available in all projects. Only site administrators or restricted site administrators can create and manage a ready-to-use role.
As a project administrator, you can use global project roles provided by the site administrators instead of creating and managing roles tailored to your projects.
Before you create a role in your project, it is a good idea to check all the available ready-to-use roles. You are likely to get ones that grant the desired set of permissions.
Global project roles serve a different purpose from that of a site-wide role. The site-wide roles enable site administrators to create restricted site administrators for providing assitance in site management. Besides that, site-wide roles can be used to grant tool/application access across the site to a user.